Method for measuring the percent consistency of pulp leaving a blow tank

ABSTRACT

The measurement and control of the percent consistency of pulp leaving a blow tank in a pulp operation is improved by using digester data and additional temperature measurements. The percent consistency of pulp is determined by using the following data: 1) the temperature and relative amounts of dry pulp, liquid and dissolved solids in a blown mixture entering the blow tank; 2) the temperature and amount of dissolved solids in a first stage filtrate liquor; and 3) the temperature of a diluted pulp mixture leaving the blow tank. Once the percent consistency of pulp is accurately measured, this value is then used to control the tonnage rate of pulp entering a first washer in a pulp washing operation.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method for improving the efficiency of a pulp washing operation by more accurately and uniformly measuring a percent consistency of pulp and thus improving the control of the tonnage rate of pulp entering a first washer in the pulp washing operation.

[0003] 2. Description of Related Art

[0004] In conventional systems, a percent consistency of pulp leaving the digester blow tank is controlled by adding the filtrate from the first stage of washing into a dilution ring around the blow tank about one third the way up from the bottom of the blow tank in an amount to produce a constant value in the amperage of the motor driving an agitator in the bottom of the blow tank. The bottom portion of this cylindrical blow tank preferably has a cone shaped bottom below the dilution ring with an agitator and discharge pipe located in the bottom of the cone. This amperage is related to the pulp consistency in the bottom portion of the blow tank. Smaller amounts of the filtrate used as diluting liquor raise the percent consistency at this point and tend to increase the load on the agitator thereby raising the amperage on the drive motor. Production rate changes also affect the percent consistency of this diluted pulp and a balance between these factors is desired.

[0005] This percent consistency of pulp leaving the blow tank is very difficult to control due to a ten or twenty minute time lag between a change in the valve opening on the filtrate line going to the dilution ring and the subsequent full effect on the amperage of the agitator that is in the bottom of the blow tank. There are several other factors that influence this amperage value other than the percent consistency of pulp thereby causing additional errors in the consistency control. Some of these factors are fiber length, the K number of the pulp, the temperature of the diluted pulp, the percent solids in the associated liquor, the relative amount of diluted pulp in the agitated section of the tank to high density pulp floating above the low density section and possibly other unknown factors. The effects any valve opening change has on the amperage of the agitator in the bottom of the tank is not only delayed by many minutes but this time lag is variable depending upon the consistency in the blow tank and the tonnage rate going through the system. All these uncompensated for variables cause the control of the percent consistency of pulp to be quite poor and the tonnage rate cannot be accurately controlled.

[0006] Some mills have an external consistency regulator following the blow tank. But, these regulators are costly and have had varying degrees of success. Many mills have not installed this type of external regulator as it is not thought to be cost effective for their mill. These regulators fail to measure or control the percent consistency of pulp and tonnage rate of the pulp going to the pulp washing operation to the degree of accuracy desired for additional control systems to optimize the pulp washing operation. These other regulators have been available for many decades but the fact that these regulators have poor efficiency have lead to the lack of widespread approval.

[0007] An excellent review of the operations and efficiencies of a pulp washing operation is given in U.S. Pat. No. 6,074,522 by Seymour and is incorporated herein by reference.

BRIEF SUMMARY OF THE INVENTION

[0008] To overcome the problems associated with prior methods of controlling the percent consistency of pulp, the present invention uses temperature measurements to accurately determine the percent consistency of pulp leaving a blow tank and this improved value is used in controlling the tonnage rate of pulp going to a first washer of a pulp washing operation. With improved uniformity and more accurately determining the percent consistency of the pulp leaving the blow tank, the efficiency of the pulp washing operation is improved. By improving the efficiency of the pulp washing operation, it becomes possible to reduce the pollution from the pulp washing operation to the waste stream from the mill. Additionally, the cost of the evaporation operation of the liquors and the cost of the bleaching operation of the pulp will be reduced. In plants without bleaching operations the more uniformly washed pulp will improve the operation of the paper machines or any other related operation. All of the associated cost factors above are related to the efficiency of the pulp washing operation, which can be improved by better control of the tonnage rate of pulp going to the first washer in the pulp washing operation. Generally better control of this pulp washing operation will also allow increased production rates through subsequent washing steps and related steps in the pulp plant.

[0009] The percent consistency of pulp going through a flow measuring device to the first washer is determined by the following variables: 1) the relative amounts of pulp, liquid and dissolved solids going into the blow tank from the digesters, along with the temperature of this blown mixture; 2) the temperature and percent dissolved solids of a first stage filtrate liquor from a first stage filtrate tank; and 3) the temperature of a diluted pulp mixture subsequently leaving the blow tank. When these variables are known, the relative proportions of material in a diluted pulp mixture going to the first washer may be calculated. Since this is not a simple mass and heat balance relationship as all variables must be accounted for including their respective heat capacities and volume ratios, the calculation for determining the percent consistency of pulp is preferably conducted using a computer program do loop to run up to 3000 cycles to find a balance that produces the conditions experienced in a diluted pulp mixture going to the flow meter, and subsequently to the first stage of washing.

[0010] In an alternate preferred embodiment, a second dilution containing an amount of first stage filtrate liquor from the first stage filtrate tank may also be added to the diluted pulp mixture leaving the blow tank. In this embodiment, the temperature of a second diluted pulp mixture after this second dilution must be determined. Since, the exact conditions of the diluted pulp mixture before this second dilution are already known and the first stage filtrate liquor from the first stage filtrate tank is the same as for the first dilution, no other measurements are required to calculate the components of this mixture. Although it is preferred that the second dilution is made using first stage filtrate liquor, a diluting liquor other than the first stage filtrate liquor may be added to the diluted pulp mixture to achieve the desired second dilution. If this diluting liquor is not the first stage filtrate liquor, the percent solids and the temperature of this diluting liquor must be determined. Additionally, a second calculation using the percent solids and temperature of the diluting liquor would have to be performed along with the previously calculated values of the diluted pulp mixture prior to the second dilution to determine the percent consistency of pulp in the second diluted pulp mixture.

[0011] In order to control the tonnage rate of dry pulp going to the first washer, it is necessary to know the percent consistency of pulp leaving the blow tank, which then goes to the flow measuring device. Once the percent consistency of pulp is known, the volume measurement for tonnage rate of pulp can be made with standard flow meters commonly used.

[0012] The heat losses in the pulp operations currently in use today are completely negligible. By way of example, this negligible heat loss is easily seen when considering that the flow rate of the first stage filtrate is generally over 5000 liters per minute and yet it travels only a few feet after the temperature is measured. Secondly, the blown pulp is never exposed to any cooling effect after flashing to atmospheric pressure into the blow tank and the temperature of the diluted pulp mixture is measured immediately as it leaves the blow tank.

[0013] Although the flow rate of the first stage filtrate liquor could be measured, it is not possible to measure the flow rate of the blown pulp as it traverses an interface line from a high consistency pulp in the upper portion of the blow tank into a lower consistency zone below the dilution ring in the blow tank. For this reason the percent consistency of the pulp leaving the blow tank cannot be determined by a ratio of the volumes being mixed therein.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

[0014] The features and advantages of the present invention will become apparent from the following detailed description of a preferred embodiment thereof, taken in conjunction with the accompanying drawings, in which:

[0015]FIG. 1 is a schematic diagram of a preferred pulp operation from a digester to a first washer; and

[0016]FIG. 2 is a block diagram showing the process by which the percent consistency of pulp is calculated.

DETAILED DESCRIPTION OF THE INVENTION

[0017]FIG. 1 shows a schematic of a preferred pulp operation from a digester 2 to a first washer 4 of a pulp washing operation. However, the present invention should not be limited to the type of pulp operation shown in FIG. 1. There are many different kinds of pulp operations which require accurate measurements of percent consistency of pulp and control of the tonnage rate of pulp. These pulp operations may contain a variety of digesters, blow tanks, pumps, flow meters, and washers. While the present invention will be described in terms of a preferred brown stock washing operation for washing cellulose, it may be applied to a variety of operations such as the washing step in bleaching plants where the drums are similar to those in a brown stock washing process. The liquid diluents used in this description may be water, recycled water, or chemical treatment agents. The material being washed or treated may be some material other than pulp. It should also be understood that various valves and pumps may be added to the pulp operation depicted in FIG. 1 as deemed necessary in the ordinary course of pulp manufacturing.

[0018] As shown in FIG. 1, wood chips are charged into the digester 2, which is preferably a steel pressure vessel that can hold 50 tons or more of the wood chips. In a preferred embodiment, a digester mixture is formed within the digester 2 by mixing wood chips with some cooking chemicals, such as sodium hydroxide and sodium sulfide solutions. In addition, some black liquor from the washing operation is preferably added to make up a total volume in the digester 2. The digester 2 is then tightly closed and steam is added to heat the digester mixture to over 300° F. and at about 100 pounds per square inch pressure. Depending on the grade of pulp desired, the digester mixture is cooked for a total time, such as two hours. The digester mixture is then blown out the bottom of the digester 2 into a blow tank 6 at atmospheric pressure. The hot digester mixture essentially explodes into the blow tank 6 releasing steam vapors and a blown mixture of fibers mixed with black liquor from dissolved lignin in the wood. The steam vapors released are sent to a heat recovery condenser to make hot water for the rest of the pulp operation or some other useful operation.

[0019] The blown mixture in the blow tank 6 is diluted with black liquor, referred to as first stage filtrate liquor, from a first stage filtrate tank 8 from the pulp washing operation and pumped to the blow tank 6 via line 10. By way of example, the consistency in the upper portion of the blow tank 6 is generally over ten percent pulp in liquor and the first stage filtrate liquor added from the first stage filtrate tank 8 via line 10 dilutes this down to about 2 to 4 percent consistency of pulp in liquor. At the bottom of the blow tank 6 is an agitator 12 which sufficiently and uniformly mixes the diluted pulp mixture. The diluted pulp mixture is then pumped via line 14 to the first washer 4. In a preferred alternate embodiment, a second dilution containing first stage filtrate liquor from the first stage filtrate tank 8 may be added to the diluted pulp mixture leaving the blow tank 14 via line 16 resulting in a second diluted pulp mixture that is sent to the first washer 4. Alternately, the second dilution may be made using a diluting liquor which is not the first stage filtrate liquor so long as the percent solids and the temperature of this diluting liquor is known. In order to determine the percent consistency of pulp in the second diluted pulp mixture, a second calculation using the percent solids and the temperature of the diluting liquor must be performed in addition to the previously calculated values of the previous diluted pulp mixture prior to the second dilution. Another important measurement for the present invention is the flow rate of the diluted pulp mixture in line 14. A flow measuring device, such as a flow meter 18, is inserted in line 14 to measure the flow rate of the diluted pulp mixture, which then establishes the tonnage rate of pulp going to the first stage of washing.

[0020] A novel feature of the present invention is the measuring of temperature data at various locations between the blow tank 6 and the first washer 4. As shown in the schematic of the preferred pulp operation in FIG. 1, temperature measurements are taken at points B, herein referred to as a filtrate temperature, and C, herein referred to as a pulp mixture temperature. In the preferred alternate embodiment, wherein a second dilution is added to the diluted pulp mixture after it leaves the blow tank, temperature measurement is also taken at point D, herein referred to as a second pulp mixture temperature.

[0021] Under steady state conditions, the percent consistency of pulp and temperature of the blown mixture, herein referred to as the blown mixture temperature, entering the blow tank 6 are essentially constant and of known values. It is also expected that the percent consistency is too high at this point for the pulp to either de-water or absorb liquor without added force. An important part of this invention is the discovery that the freshly blown mixture in the upper portion of the blow tank 6 is at a constant relative temperature that can be determined from data from the digester charging. A simple measurement of this temperature, without the digester data for determining the ratios and flow rates of pulp, liquids and dissolved solids in the top portion of the blow tank, would be of no use for determining the percent consistency of pulp out of the blow tank. The amount of chips charged into the digester is weighed over a weightometer, the volume and concentration of cooking liquor is measured for each digester charge, and the amount of black liquor makeup to produce a given total volume in each digester charge is measured. The amount of steam used in direct steamed cooks is easily calculated, as is the amount of water that is lost to the heat recovery system from the digester blow. Except for special cases of cold blow systems the pulp from the digesters is quite hot and under a high pressure. This pulp is blown into the blow tank 6 at atmospheric pressure and the resulting temperature then depends only upon the percent dissolved solids in the associated liquor. The cooking cycle depends upon these measured values to produce a uniform product. A portion of this invention is the discovery that the temperature of the blown mixture entering the blow tank could be accurately determined from the measured values to the digester charge and additionally the amount of dry pulp associated with this liquor could also be obtained from measured values and the pulp yield to wood ratio from past experience. This yield value changes very little and is sufficiently accurate for this calculation. Generally, the black liquor produced in cooking the wood chips into pulp is up to about nineteen percent solids in the liquor going to the blow tank from the digester. This high concentration has a significant effect on the temperature when blown to atmospheric pressure and also in the heat balance calculations for percent consistency of pulp determination and control.

[0022] As can be seen from the above discussion, the calculation to determine the percent consistency of pulp is not a simple mass and heat balance relationship as all variables must be accounted for including their respective heat capacities and volume ratios. In practice, the calculations to arrive at the percent consistency of the pulp mixture leaving the blow tank requires a computer program do loop to run up to 3000 cycles to find the balance that produces the conditions experienced leaving the blow tank and in the final pulp mixture.

[0023] The filtrate and pulp mixture temperature measurements corresponding to points B and C must be quite accurate and the preferred specifications for these instruments call for repeatable accuracy to 0.005 degrees centigrade. Preferably, the instruments used to measure the temperature are special paired resistance thermometers, calibrated together as a pair in the range they are to be operating, generally 85-95° C. When using the preferred alternate embodiment in which first stage filtrate liquor is added to the diluted pulp mixture, the percent consistency of pulp at the flow meter and in the vat of the first washer is readily determined since the diluting liquid is from the same source. However, if the diluting liquor is not first stage filtrate liquor, a second calculation must be conducted using the percent solids and temperature of the diluting liquor in conjunction with the previously calculated values of the diluted pulp mixture prior to the second dilution.

[0024] Using the alternate preferred embodiment, a percent consistency in the first washer 4 is determined by measuring a second pulp mixture temperature after the second dilution point. As an extra benefit this procedure can be used to determine the volumes of the first stage filtrate liquors added into the blow tank 10 and also into the second dilution point 16 by measuring the pulp mixture and second pulp mixture temperatures at points C and D. Combined with the reading from the flow meter 18, the flow rate in liters per minute can be determined for each of these separate streams.

[0025] The consistency of the pulp leaving the blow tank may be controlled by adjusting the amount of the first stage filtrate liquor entering the blow tank 10. The desired percent consistency of pulp is then maintained and calculated as discussed herein. However, due to the long delay times a preferred method for controlling the production rate to the entire pulp washing operation is to hold the amount of first stage filtrate liquor entering the blow tank at a constant value, so long as the percent consistency is within acceptable limits, and regulating only the flow rate of the diluted pulp mixture of known consistency to give a constant tonnage rate of dry pulp to the washing operation.

[0026] The preferred method for calculating a final percent consistency is shown in FIG. 2. As shown in FIG. 2, the amount of dissolved solids and the filtrate temperature of the first stage filtrate liquor are measured. Then, the relative amounts of dry pulp, liquid and dissolved solids in the blown mixture, along with the blown mixture temperature, are determined. Then, the pulp mixture temperatures are measured. Next, the percent consistency of pulp (x) is calculated for the diluted pulp mixture. Then, a calculated temperature, T1, for the diluted pulp mixture is determined. If T1 is greater than the measured pulp mixture temperature, the percent consistency of pulp is x. If T1 is not greater than the measured pulp mixture temperature, the blown mixture stream is increased and the steps repeated until T1 is greater than the measured pulp mixture temperature. It is preferred that a computer program be used in order to determine the final percent consistency (x). Preferably, the calculations are determined by starting with an initial diluted pulp mixture consisting of sufficient blown pulp and liquor mixed with a given volume of first stage filtrate liquor to give a percent consistency of pulp of 0.005 percent (0.00005 for x). A heat balance is made with this condition and then the percent consistency of pulp is preferably incremented by units of 0.005% for each following cycle. Checks are made at each level of percent consistency to see if it matches one of the actual conditions experienced. When a calculated temperature, T1, from the heat balance exceeds the diluted pulp mixture temperature measured by the temperature instruments at the flow meter, the percent consistency value is set for the pulp mixture at the flow meter. This process for determine the percent consistency of pulp may be used at any point in this system from the blow tank to the first stage washer where the temperature of the mixture is measured after the first stage filtrate liquor is added.

[0027] Once the percent consistency of pulp (% CONS) at the position of the flow meter has been determined the preferred equations to determine a dry pulp flow rate in metric tons per hour (MTPH) and to set a desired flow rate in liters per minute (LPM) to produce the desired tonnage rate of pulp are as follows:

[0028] Tonnage production rate in metric tons per hour (MTPH)

MTPH=LPM*% CONS*0.0006  EQUATION 1

[0029] Or rearranged, the flow rate to get a given tonnage rate is:

LPM=MTPH*1666.7/% CONS  EQUATION 2

[0030] Much of the digester data changes very slowly and is preferably retained in an active program and the actual temperature data is preferably read every minute directly from the system computer files giving a continuous readout of the percent consistencies and a continuously calculated set point for the flow rate to produce the desired tonnage rate for the system.

[0031] The difference in liquor volume between a system that provides for a constant percent consistency and the present invention which regulates the tonnage rate of pulp by regulating the flow rate of a known percent consistency will be made up by a vat dilution flow, which is regulated by a vat level controller and drum speed. Since this dilution is done with the same liquor from the first stage filtrate tank the system remains in balance the same as if all the dilution had been made in the blow tank. With the drum speed remaining constant the hydraulic capacity of the system remains constant and the total volume of liquor added to the pulp going to the first washer per metric ton of pulp is constant and the vat consistency should also be constant.

[0032] The blowing schedule of the digesters establishes the average production rate and changes in this schedule must be adjusted for in the level control of the blow tank. The digesters blow into the top portion of this blow tank intermittently but on a rigid time schedule most of the time so the rate of production leaving this blow tank can be set at a quite constant value. Level control in the blow tank can be done either by the operator or automatically by computer with sufficient inputs. One of the advantages of this system is that the tonnage rate of pulp is controlled better and therefore the level control in the blow tank is more easily controlled by matching the production rate with the digester spacing schedule and the known tonnage yield from the digesters.

[0033] A shower flow to achieve a desired percent solids going to the evaporators is then set by estimating the consistency off the last washer and applying a shower flow to achieve a given dilution factor. This dilution factor is then adjusted up or down to achieve the desired percent solids in the liquor going to the evaporators. An equation has been developed to equate the actual dilution factor and the percent solids.

[0034] The definition of the dilution factor used herein is the standard definition used in the pulp and paper industry. This is a dimensionless number that represents the weight of the shower water that goes all the way through the pulp mat in a washing operation divided by the weight of dry fibers in the mat in any unit time.

[0035] Among other ways to determine the dilution factor Equation 3 has been developed by determining all the solids and liquids coming from the digesters then calculating the amount of water required to add to that to produce the percent solids experienced. This amount of water divided by the equivalent weight of dry pulp is the dilution factor.

Dilution Factor=−8.368+173.12/P−178.3/(P*P)  EQUATION 3

[0036] WHERE: P is the percent solids in the first stage filtrate liquor

[0037] The percent solids in the liquor from the first stage filtrate tank can be determined by either instruments or by manual tests. Adjustments to the dilution factor for percent solids determination need not be made more than once or twice per eight-hour shift. This equation can be adjusted to fit any one particular pulp washing operation by using the method listed herein and actual data from that particular mill. Recycled liquors to the digester, blow tank, or first washer vat are not a source of any additional water or solids to the overall system and do not influence the above equation.

[0038] The dilution factor is generally the basic property in the pulp washing operation that is correlated with the soda loss, heat loss, water usage, liquor carryover into subsequent operations, evaporator costs, pollution control, chemical usages such as anti-foam agents and reduced bleaching chemicals usage, and other parameters used in the economic optimization of the pulp washing operation. With the present invention allowing improvements in the control of the tonnage rate and associated factors and probable increased average production rates, the economic optimization with these factors are therefore a part of this application.

[0039] It should be recognized and appreciated that exact numerical values for any of these equations are not required for near perfect control of the pulp washing operation using the present method. The constancy of the operations dictated by these strict control procedures are the determining factors in gaining the most efficient and economically optimum operational set points.

[0040] Although the present invention has been disclosed in terms of a preferred embodiment, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention as defined by the following claims: 

What is claimed is:
 1. A process for measuring the consistency of pulp in a pulp operation, said pulp operation comprising at least a blow tank, a first washer, and a first stage filtrate tank, comprising a) determining a percent dissolved solids in a first stage filtrate liquor entering said blow tank from said first stage filtrate tank; b) measuring a filtrate temperature of said first stage filtrate liquor; c) determining the relative amounts of dry pulp, liquid and dissolved solids in a blown mixture going into said blow tank; d) determining a blown mixture temperature of said blown mixture going into said blow tank; e) measuring a pulp mixture temperature from a diluted pulp mixture leaving said blow tank; and e) calculating a percent consistency of pulp in said diluted pulp mixture using said percent dissolved solids in said first stage filtrate liquor, said filtrate temperature, said relative amounts of dry pulp, liquid and dissolved solids in said blown mixture, said blown mixture temperature, and said pulp mixture temperature; f) calculating a calculated temperature of said diluted pulp mixture using said percent dissolved solids in said first stage filtrate liquor, said filtrate temperature, said relative amounts of dry pulp, liquid and dissolved solids in said blown mixture, said blown mixture temperature, and said pulp mixture temperature; and g) adding an additional amount of said first stage filtrate liquor or said blown mixture to increase said percent consistency of pulp by an incremental amount; and h) determining a percent consistency of pulp by repeating steps a)-g) until said calculated temperature is greater than said pulp mixture temperature.
 2. The method of claim 1, further comprising measuring a flow rate of said diluted pulp mixture.
 3. The method of claim 2, further comprising determining a tonnage rate of pulp in said diluted pulp mixture using said percent consistency of pulp and said flow rate of said diluted pulp mixture.
 4. The method of claim 1, wherein said filtrate temperature and said pulp mixture temperature are measured within 0.005 degrees centigrade.
 5. The method of claim 1, wherein said percent dissolved solids in said first stage filtrate liquor is constant.
 6. The method of claim 1, wherein said filtrate temperature is constant.
 7. The method of claim 1, further comprising measuring a second pulp mixture temperature after the introduction of a second diluting liquor to said diluted pulp mixture leaving said blow tank.
 8. The method of claim 7, wherein said second diluting liquor is said first stage filtrate liquor.
 9. The method of claim 7, further comprising determining a temperature and a percent solids in said second diluting liquor.
 10. The method of claim 7, further comprising measuring a flow rate of said diluted pulp mixture.
 11. The method of claim 10, further comprising determining a tonnage rate of pulp in said diluted pulp mixture using said percent consistency of pulp and said flow rate of said diluted pulp mixture.
 12. The method of claim 7, wherein said filtrate temperature, said pulp mixture temperature, and said second pulp mixture temperature are measured within 0.005 degrees centigrade.
 13. The method of claim 7, wherein said percent dissolved solids in said first stage filtrate liquor is constant.
 14. The method of claim 7, wherein said filtrate temperature is constant. 